More efficient `expm`s

[ChrisRackauckas]

Base.expm(L::DiffEqArrayOperator) = expm(L.α.coeff*L.A)

We don't want to overwrite the matrix because then calculations further down the line will be incorrect, but we can make this more efficient via something like:

scale!(L.A,L.α.coeff)
out = expm(L.A)
scale!(L.A,inv(L.α.coeff))
out

That'll get rid of a matrix allocation. We might need to pull this off other places as well.